Molecular identification and studies on genetic diversity and structure-related GC heterogeneity of Spatholobus Suberectus based on ITS2

To determine the role of internal transcribed spacer 2 (ITS2) in the identification of Spatholobus suberectus and explore the genetic diversity of S. suberectus. A total of 292 ITS2s from S. suberectus and 17 other plant species were analysed. S. suberectus was clustered separately in the phylogenetic tree. The genetic distance between species was greater than that within S. suberectus. Synonymous substitution rate (Ks) analysis revealed that ITS2 diverged the most recently within S. suberectus (Ks = 0.0022). These findings suggested that ITS2 is suitable for the identification of S. suberectus. The ITS2s were divided into 8 haplotypes and 4 evolutionary branches on the basis of secondary structure, indicating that there was variation within S. suberectus. Evolutionary analysis revealed that the GC content of paired regions (pGC) was greater than that of unpaired regions (upGC), and the pGC showed a decreasing trend, whereas the upGC remained unchanged. Single-base mutation was the main cause of base pair substitution. In both the initial state and the equilibrium state, the substitution rate of GC was higher than that of AU. The increase in the GC content was partly attributed to GC-biased gene conversion (gBGC). High GC content reflected the high recombination and mutation rates of ITS2, which is the basis for species identification and genetic diversity. We characterized the sequence and structural characteristics of S. suberectus ITS2 in detail, providing a reference and basis for the identification of S. suberectus and its products, as well as the protection and utilization of wild resources.